Implantable medical devices incorporating plasma polymerized film layers and charged amino acids

ABSTRACT

An implantable medical device, such as a stent, is disclosed comprising an amino acid or a polypeptide bonded to a plasma polymerized film layer formed on the device. A method of manufacturing the same is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of medical devices, such as stents.More particularly, this invention is directed to coatings which includechemically-bound polymers and/or oligomers of L-arginine.

2. Description of Related Art

In the field of medical technology, there is frequently a necessity toadminister drugs locally. To provide an efficacious concentration to thetreatment site, systemic administration of medication often producesadverse or toxic side effect for the patient. Local delivery is apreferred method in that smaller total levels of medication areadministered in comparison to systemic dosages, but are concentrated ata specific site. Thus, local delivery produces fewer side effects andachieves more effective results.

A commonly employed technique for the local delivery of a drug isthrough the use of a medicated stent. One method of medicating a stentis by coating the stent with a polymer having a drug incorporatedtherein. L-arginine, or polypeptide oligomeric derivatives thereof, forexample, those containing 5 to 20 amino acid units, is one example of asubstance that can be delivered via a stent.

L-arginine is known to be a precursor of endothelium derived nitricoxide (NO). NO is synthesized from L-arginine, or its polymeric and/oroligomeric derivatives, by the enzyme NO synthase oxygenase, ahomodimeric flavo-hemoprotein that catalyzes the 5-electron oxidation ofL-arginine to produce NO and L-citrulline. Among other therapeuticproperties, NO relaxes vascular smooth muscle cells and inhibits theirproliferation. References describing beneficial therapeutic propertiesof L-arginine include U.S. Pat. No. 5,861,168 to Cooke et al. Cooke etal. teach that administering L-arginine, as the NO precursor, restoresvascular NO activity in patients with endothelial vasodilatordysfunction due to restenosis. Moreover, Uemura et al. (Circulation,vol. 102, 2629-2635 (2000)), teach that the heptamer of L-arginine (R7)exhibits biological activity in the reduction of neointimialhyperplasia.

According to traditional techniques, L-arginine or its polymers and/oroligomers can be physically incorporated into a polymeric matrix for insitu local delivery. The embodiments of the present invention providealternative methods for local delivery of L-arginine, or its polymersand/or oligomers, by an implantable device such as a stent.

SUMMARY

An implantable medical device is provided, comprising a plasmapolymerized film layer and a polypeptide chemically bonded to the plasmapolymerized film layer. The device can be, for example, a stent, such asa balloon expandable or self-expandable stent. The plasma polymerizedfilm layer can be derived from an organic substance having carboxylgroups. The organic substance can include a low molecular weight organicacid such as acrylic acid, propionic acid, butyric acid, valeric acid,and methacrylic acid. Representative examples of the polypeptide caninclude poly(L-arginine), poly(D-arginine), poly(L-lysine),poly(D-lysine), poly(δ-guanidino-α-aminobutyric acid), or a racemicmixture of poly(L-arginine) or poly(D-arginine). In accordance with oneembodiment, a surface of the device on which the plasma polymerized filmlayer is deposited includes a carbon deposit, wherein the plasmapolymerized film layer is formed over and is bonded to the carbondeposit.

A method of forming a coating for an implantable medical device is alsoprovided, comprising forming a plasma polymerized film layer on thedevice and bonding a polypeptide to the plasma polymerized film layer.In one embodiment, the method can additionally include implanting carbondeposits in the surface of the device prior to forming the plasmapolymerized film layer, wherein the plasma polymerized film layer isformed on carbon deposits. The bonding of the polypeptide to the plasmapolymerized film layer can be conducted by exposing the plasmapolymerized film layer to a free base form of the polypeptide.

A stent comprising a plasma polymerized film layer and an amino acidionically bonded to the plasma polymerized film layer is also provided.

A method of treatment of a patient having a vascular occlusion is alsoprovided, comprising implanting a stent at the site of the occlusion,the stent including a plasma polymerized film layer having a firstcharge and administering to the patient a polypeptide having an opposingsecond charge wherein some of the polypeptide circulating through theblood stream of the patient will couple to the plasma polymerized filmlayer. The polypeptide can be administered orally, intravenously, or bya drug delivery catheter.

DETAILED DESCRIPTION

L-arginine, commonly abbreviated as “R” or “Arg,” also known as2-amino-5-guanidinovaleric acid, has a formulaNH═C(NH₂)—NH—(CH₂)₃—CH(NH₂)—COOH. L-arginine is an amino acid. Due tothe presence of a strong basic guanidinium group, —NH—C(NH₂)═NH,carrying a partially uncompensated positive charge, L-arginine, itspolymers and/or oligomers are highly cationic. For example, the heptamerof L-arginine has a pK_(a) of 13.2.

Polymers and/or oligomers of L-arginine that can be used are referred toherein as “PArg.” PArg are polycationic peptides comprising a pluralityof repeating monomeric amino acid units and have a general formulaH—[NH—CHX—CO]_(p)—OH, where “p” can be within a range of 5 and 1,000,typically, within a range of between 5 and 20. For example, a heptamer(designated R7) or a nonamer (R9), having p=7 and p=9, respectively, canbe used. In the formula of PArg, “X” is 1-guanidinopropyl radical havingthe structure —CH₂—CH₂—CH₂—NH—C(NH₂)═NH. The terms “polymers and/oroligomers of L-arginine” and “PArg” are intended to include L-argininein both its polymeric and oligomeric form.

In addition to PArg, other polycationic peptides can be alternativelyused. Examples of such alternative polycationic peptides includepoly(L-arginine), poly(D-arginine), poly(L-lysine), poly(D-lysine),poly(δ-guanidino-α-aminobutyric acid), racemic mixtures ofpoly(L-arginine) and poly(D-arginine), and chitosan.

The modification of the surface of the substrate, for example a surfaceof a stent, can include the following steps:

-   -   (a) implanting of carbon into the stent surface;    -   (b) functionalizing the stent surface containing the implanted        carbon; and    -   (c) neutralizing the functionalized stent surface.

Prior to the implantation of carbon, the outer surface of the stent canbe cleaned by, for example, argon plasma treatment or any other suitablecleaning method so as to remove contaminants and impurities that areintroduced during the manufacturing process. To implant carbon into thesurface, in accordance with one method, the stent can be treated withmethane plasma in a Plasma Ion Implantation Unit (PIIU). PIIU is asystem known to those having ordinary skill in the art. The methaneplasma treatment can be conducted by enclosing the stent inside astainless steel mesh followed by generating the methane plasma using aradio frequency power source. The length of time needed implant thenecessary amount of carbon into the stent surface can be between about 2to 5 minutes. Process parameters for the methane plasma that can beemployed are listed in Table 1 below. TABLE 1 The Parameters of theMethane Plasma for Implantation of Carbon into the Stent Surface (>99.9%by Volume of Methane) Parameter Parameter Range Exemplary Value Methanegas flow rate (sccm) 10 to 200 30 Volume of the PIIU chamber (cm³) —2,000 Pressure (mTorr) 0.1 to 2.0 0.5 RF power (Watts) 10 to 1,000 100RF frequency (MHz) 2 to 2,800 13.56 Bias voltage--stent (kV) −10 to −80−50 Pulse width--stent (μs) 5 to 100 20 Frequency--stent (Hz) DC to2,000 200

Alternatively, the stent can be implanted with carbon by sputteringcarbon from a carbon cage electrode. The stent can be placed inside thecarbon cage electrode, which in turn can be put inside a plasma chamber.A plasma, for example, argon plasma, can be generated using a radiofrequency power source, followed by applying a bias voltage to thecarbon cage. In addition to argon, the gaseous medium can also includeother components such as nitrogen, for example, 1:1 by volume blend ofargon and nitrogen. The length of time needed implant the necessaryamount of carbon into the stent surface can be between about 2 to 5minutes. Process parameters for the argon plasma that can be employedare listed in Table 2. Under the described conditions, carbon can besputtered from the carbon cage, accelerated towards the stent surface,and implanted into the stent surface. TABLE 2 The Parameters of theArgon Plasma for Implantation of Carbon into the Stent Surface bySputtering Carbon from a Carbon Electrode (>99.9% by Volume of Argon)Parameter Parameter Range Exemplary Value Gas flow rate (sccm) 10 to 50050 Volume of the PIIU chamber (cm³) 2,000 Pressure (mTorr) 0.1 to 500 50RF power (Watts) 10 to 1,000 200 RF frequency (MHz) 2 to 2,800 13.56Bias voltage--stent (kV) −5 to −30 −10 Bias voltage-carbon electrode(kV) 15 to 20 20 Pulse width--stent (μs) 5 to 20 20 Frequency--stent(Hz) DC-2,000 500 Bias voltage-grid of the PIIU (V) −300 to −5,000−1,000

Subsequent to the implantation of the carbon deposit, the stent can becleaned using argon plasma. Process parameters for the argon plasmacleaning that can be employed are listed in Table 3. TABLE 3 TheParameters of the Argon Plasma for Cleaning the Stent (>99.9% by Volumeof Argon) Process Parameter Range Exemplary Value Argon flow rate (sccm)10 to 250 250 Pressure (mTorr) 10 to 250 230 RF power (W) 50 to 450 400RF frequency (MHz) 2 to 2800 13.54 Time (minutes) 3 to 30 5

After the stent surface is implanted with carbon and cleaned asdescribed above, or by using any other acceptable method of carbonimplantation and cleaning known to those having ordinary skill in theart, the stent surface can be functionalized by plasma polymerization asis understood by those having ordinary skill in the art. Regardless ofwhich method is used for implanting carbon into the stent surface,carbon dioxide/acrylic acid plasma can be used for functionalization.

To functionalize, the carbon-implanted stent surface can be exposed tocarbon dioxide/acrylic acid plasma to form an acrylate or acrylate-likepolymer film layer on the surface of the stent. The carbon deposit onthe surface of the stent provides a site at which covalent bonds can beformed with the plasma deposited acrylate or acrylate-like polymer filmlayer. One having ordinary skill in the art will recognize that somefragmentation of the acrylate can typically occur during the plasmapolymerization process, resulting in an acrylate-like polymer layer offragmented acrylate being formed on the carbon-implanted stent surface.

The plasma can be generated using a radio frequency power source underthe conditions shown in Table 4. Instead of acrylic acid, those havingordinary skill in the art may select another low molecular weightsaturated or unsaturated organic acid, for example, propionic acid,butyric acid, valeric acid, methacrylic acid, or mixtures thereof. TABLE4 The Conditions for Conducting Functionalization of the Stent Surfaceby Plasma Polymerization (CO₂/Acrylic Acid Plasma) Exemplary ParameterParameter Range Value Acrylic acid gas rate (ml/min) 0.05-0.35  0.2Carbon dioxide gas flow rate (sccm) 60-200 90 Power (W) 10-300 100Volume of the PIIU chamber (cm³) — 2,000 Pressure (mTorr) 70 to 250 150RF power (Watts) 50 to 250 100 RF frequency (MHz) 2 to 2,800 13.56

Carbon dioxide and acrylic acid can be pre-mixed by combining theirrespective streams flowing at rates shown in Table 4, and the combinedstream can be fed into the PIIU chamber. Carbon dioxide can be suppliedwith acrylic acid to limit the rate of de-carboxylation which can occurwith an organic acid in a plasma field. Acrylic acid can be vaporizedprior to being combined with carbon dioxide. To vaporize acrylic acid,vacuum can be employed, for example, about 50 mTorr vacuum, and acrylicacid can be introduced by using, for example, a syringe pump.

In another embodiment, carbon dioxide and vaporized acrylic acid can befed into the PIIU chamber as separate streams, at a flow rate for eachrespective stream as shown in Table 4.

The acrylic acid plasma can be applied for about 10 minutes, the timelimit being dependent on the desired thickness of the acrylate oracrylate-like polymer film layer. The thickness of the plasma polymerfilm layer can be about 20 nm to about 500 nm, more narrowly about 70 nmto about 150 nm, for example, about 125 nm. In accordance with anotherembodiment, a pulsed plasma condition, as is understood by one ofordinary skilled in the art, can be employed for the deposition ofpolymer film layer. The process parameters are similar to those shown byTable 4, but for the power range being between about 60 W to about 450W, for example, between about 250 W and about 350 W. For theimplementation of pulsed plasma, the RF power can be pulsed at about 500to 5,000 Hz, for example 1,000 Hz to about 1,250 Hz, using, for example,a square wave pulse sequence. The duty period, the time in which thepower is on, can be between 15% and 100%, for example, 20% to 35%. Withthe use of pulsed plasma condition, the rate of de-carboxylation can befurther limited.

Following deposition of the plasma-polymerized film layer, the plasmafield can be purged with argon without an applied RF field to allowsurface free radicals to be quenched by recombination prior to exposureto atmospheric oxygen. Table 5 provides parameters for this quenchingprocess: TABLE 5 The Conditions for Recombination of the Surface FreeRadicals Process Parameter Range Exemplary Value argon — (>99.9% byvolume) Gas flow rate (sccm) 30 to 300 230 Pressure (mTorr) 50 to 500250 Time (minutes) 2 to 10 3

Following plasma polymerization, the carboxylated stent surface can beneutralized. To neutralize, the stent can treated with a dilutedalkaline solution, for example, sodium hydroxide solution. The durationof treatment can be about 30 minutes. Following the process ofneutralization, the stent can be washed with de-ionized water to removethe residual sodium hydroxide solution.

The neutralization procedure described above completes the process ofmodification of the stent surface. At this point, PArg, for example, R7can be incorporated onto the stent by ionic coordination. To incorporateR7, the stent can be placed in a container containing an excessiveamount of aqueous solution of R7, for example, the R7 solution in a freebase form.

To obtain R7 in a free base form, R7 can be dissolved in water and astrong alkali can be added, such as potassium hydroxide KOH, raising thepH of the R7 solution to about 13. In a strongly alkaline environment,the guanidinium fragments of R7 are de-protonated and R7 in a free baseform can be obtained as a result. In the free base form, R7 is a cationhaving the positive charge mainly concentrated on the imino nitrogen ofthe guanidinium group of R7 (NH=fragments). Such cation for the purposesof this invention is designated as R7⁺.

When the carboxylated stent surface containing neutralized polyanionicpoly(acrylic acid)-like material is brought in contact with the aqueoussolution of R7, the macromolecules of the poly(acrylic acid)-likematerial get solvated followed by charge separation, for example bydissociation, leading to the creation of carboxyl-anions as shown byreaction (I).

where X is a carbonized structure on the carbon-implanted stent surfaceto which poly(acrylic acid)-like material is attached.

R7⁺ cations are then ionically coordinated around negatively chargedcarboxyl-anions. Such coordination can be schematically shown byreaction (II):

Alternatively, R7 in a non-free base form can be also coordinated aroundthe carboxyl-anions. Although the cationic nature of R7 in the non-freebase form is not as pronounced as it is in the free base form, thepositive charge on the guanidinium group is sufficient to causecoordination of R7 around the carboxylated stent surface.

When the modified neutralized stent is brought in contact with the R7solution, the solution can be gently agitated, for example, by stirringfor about 30 minutes, followed by rinsing the stent with de-ionizedwater and drying. Optionally, a polymeric topcoat can be then applied,examples of suitable polymers being poly(ethylene-co-vinyl alcohol) andPoly(butyl methacrylate).

According to another embodiment, when R7 is brought in contact with theneutralized modified stent surface containing carboxyl groups, R7 can betrapped by being grafted to the carboxyl groups-containing stent surfaceby covalent conjugation. The carboxyl groups will react with aminogroups of R7 to form an amide. One possible path of reaction can beillustrated by reaction (III) and the conditions of the reaction will beselected by those having ordinary skill in the art:

According to yet another embodiment, the stent, modified and neutralizedas described above, can be placed at a diseased site in a blood vessel.Due to the polyanionic nature of the poly(acrylic acid) coating, thestent coating will carry a negative charge. R7 is then administeredsystemically, for example, intravenously, orally or through a perfusionballoon. R7 will be carried through the circulatory system and when R7approaches the stent, some of positively charged R7 will coordinatearound the negatively charged stent surface to form an ionic complexbetween R7⁺ and the poly(acrylic acid)-based anion, thus trapping R7.After untrapped R7 is cleared from the circulatory system, trapped R7will still persist for some time providing an enhanced concentration ofR7 at the vascular injury site.

The method of the present invention has been described in conjunctionwith a stent. The stent can be used in any part of the vascular system,including neurological, carotid, coronary, renal, aortic, iliac, femoralor any other peripheral vascular system. The stent can beballoon-expandable or self-expandable. There are no limitations on thesize of the stent, its length, diameter, strut thickness or pattern.

The use of the coating is not limited to stents and the coating can alsobe used with a variety of other medical devices. Examples of theimplantable medical device, that can be used in conjunction with theembodiments of this invention include stent-grafts, grafts (e.g., aorticgrafts), artificial heart valves, cerebrospinal fluid shunts, pacemakerelectrodes, axius coronary shunts and endocardial leads (e.g., FINELINEand ENDOTAK, available from Guidant Corporation). The underlyingstructure of the device can be of virtually any design. The device canbe made of a metallic material or an alloy such as, but not limited to,cobalt-chromium alloys (e.g., ELGILOY), stainless steel (316L), “MP35N,”“MP20N,” ELASTINITE (Nitinol), tantalum, tantalum-based alloys,nickel-titanium alloy, platinum, platinum-based alloys such as, e.g.,platinum-iridium alloy, iridium, gold, magnesium, titanium,titanium-based alloys, zirconium-based alloys, or combinations thereof.Devices made from bioabsorbable or biostable polymers can also be usedwith the embodiments of the present invention.

“MP35N” and “MP20N” are trade names for alloys of cobalt, nickel,chromium and molybdenum available from Standard Press Steel Co. ofJenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20%chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20%nickel, 20% chromium, and 10% molybdenum.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects. Therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit and scope of this invention.

1-15. (canceled)
 16. A stent comprising a plasma polymerized film layerand a substance comprising a charged amino acid group, wherein thesubstance is attached to the plasma polymerized film layer. 17.(canceled)
 18. (canceled)
 19. The stent of claim 16, wherein thesubstance comprises L-arginine, D-arginine, L-lysine, D-lysine,δ-guanidino-α-aminobutyric acid, or combinations thereof.
 20. The stentof claim 16, wherein the amino acid group comprises a positive charge.21. The stent of claim 16, wherein the amino acid group comprises anegative charge.
 22. The stent of claim 16, wherein the substancecomprises chitosan.
 23. The stent of claim 16, wherein the substancecomprises a drug.
 24. The stent of claim 16, wherein the attachmentcomprises an ionic bond between the substance and the plasma polymerizedfilm layer.
 25. The stent of claim 16, wherein the stent comprises acarbon deposit on the surface of the stent, and wherein the carbondeposit is attached to the plasma polymerized film layer.
 26. A methodof forming a stent comprising a drug, wherein the method comprisesfunctionalizing a surface of the stent with a plasma polymerized filmlayer; and attaching a substance comprising a charged amino acid groupto the plasma polymerized film layer, wherein the substance comprises adrug.
 27. The method of claim 26, wherein the substance comprisesL-arginine, D-arginine, L-lysine, D-lysine, δ-guanidino-α-aminobutyricacid, or combinations thereof.
 28. The method of claim 26, wherein theamino acid group comprises a positive charge.
 29. The method of claim26, wherein the amino acid group comprises a negative charge.
 30. Themethod of claim 26, wherein the substance comprises chitosan.
 31. Themethod of claim 26, wherein the attachment comprises an ionic bondbetween the substance and the plasma polymerized film layer.
 32. Themethod of claim 26, wherein the plasma polymerized film layer isattached to a carbon deposit on the surface of the stent.
 33. A methodof treating a vascular condition, wherein the method comprisesimplanting a stent comprising a plasma polymerized film layer and asubstance comprising a charged amino acid group, wherein the substanceis attached to the plasma polymerized film layer.
 34. The method ofclaim 33, wherein the vascular condition comprises an occlusion, aninjury, a vasodilator dysfunction, restenosis, or a combination thereof.35. The method of claim 33, wherein the substance comprises L-arginine,D-arginine, L-lysine, D-lysine, δ-guanidino-α-aminobutyric acid, orcombinations thereof.
 36. The method of claim 33, wherein the amino acidgroup comprises a positive charge.
 37. The method of claim 33, whereinthe amino acid group comprises a negative charge.
 38. The method ofclaim 33, wherein the substance comprises chitosan.
 39. The method ofclaim 33, wherein the substance comprises a drug.
 40. The method ofclaim 33, wherein the attachment comprises an ionic bond between thesubstance and the plasma polymerized film layer.
 41. The method of claim33, wherein the stent comprises a carbon deposit on the surface of thestent, wherein the carbon deposit is attached to the plasma polymerizedfilm layer.
 42. A stent comprising a plasma polymerized film layer and asubstance comprising an amino acid, wherein the substance is chemicallybonded to the plasma polymerized film layer.
 43. The stent of claim 42,wherein the plasma polymerized film layer is derived from an organicsubstance having carboxyl groups.
 44. The stent of claim 43, wherein theorganic substance comprises a low molecular weight organic acid or acombination of low molecular weight organic acids.
 45. The stent ofclaim 44, wherein the organic substance comprises a component selectedfrom a group consisting of acrylic acid, propionic acid, butyric acid,valeric acid, methacrylic acid, and combinations thereof.
 46. The stentof claim 42, wherein the substance comprises L-arginine, D-arginine,L-lysine, D-lysine, δ-guanidino-α-aminobutyric acid, or a mixturethereof.
 47. The stent of claim 46, wherein the L-arginine is releasablefor local delivery in a patient.
 48. The stent of claim 42, wherein asurface of the stent on which the plasma polymerized film layer isdeposited includes a carbon deposit, wherein the plasma polymerized filmlayer is formed on and bonded to the carbon deposit